1. Field of the Invention
This invention relates to the detection of fingerprints and the like, and in particular to the enhancement of lipid fingerprints on porous and other surfaces.
Sweat glands are located all over the body except for the soles of the feet whereas sebaceous glands are found primarily on the face, neck, upper chest and upper back. Although only secreted over a relatively small area of the skin the sebum (lipids), which are viscous liquids at the skin surface temperature of 32.degree. C., will diffuse over the entire area. Fingerprints are comprised of sweat and sebum in varying quantities. Although sebum is not directly secreted onto the fingers it is easily transferred in significant amounts by frequent contact of the hands and fingers with the face. The exact composition of the fingerprint not only varies from one individual to the next but will also vary for each individual depending upon many uncontrolled factors such as the time of day, the time period since washing the hands, the temperature, whether the individual frequently touches their face with their hands as well as any circumstances that may lead to excessive sweating such as stress or physical exercise.
Fingerprints that have been deposited onto surfaces that are then subjected to water will only be comprised of lipids since the water removes the water soluble components such as amino acids and leaves the water insoluble components such as lipids. In these circumstances the fingerprint can only be visualised using a lipid detection method.
Fingerprints that have been deposited onto cocaine exhibits are extremely difficult to visualize since the drug reacts with cyanoacrylate which is the chemical treatment most commonly used for the plastic packaging in which cocaine is usually contained. The use of chemicals for the visualisation of amino acids is restricted to porous surfaces excluding plastics. The fingerprint may be visualised using a lipid detection technique which does not interact with the cocaine.
2. Description of the Prior Art
A number of techniques for the detection of fingerprints have been documented.
Fingerprints developed by 1,8-diazofluoren-9-one (DFO) and Ninhydrin (followed by zinc chloride toning) on porous surfaces such as paper and cardboard are well recorded in the literature. For example, see The Use of 1,8-diazafluoren-9-one (DFO) for the Fluorescent Detection of Latent Fingerprints on Paper, a Preliminary Evaluation, Pounds et al., Journal of Forensic Sciences, JFSCA, vol. 35, No. 1, January, 1990. More specifically, these treatments react with the amino acid content of the fingerprint resulting in fluorescent images of the ridges which tend to be spotty in appearance. The ridges of the fingerprint are lined with sweat pores and the amino acids are contained within the sweat component of the fingerprint which explains the spotty pattern of fingerprints visualised by these methods.
One drawback with these fluorescent dyes is the requirement for an expensive light source in the price range of $10-20,000 for forensic lamps. Such costly equipment restricts the availability of this technology to police forces which have large budgets.
Another drawback is the spotty nature of the images obtained with amino acid detection techniques. The interpretation of the ridge detail and the comparison to a known sample is made that much harder when the ridges of the fingerprint do not appear as a continuous image. A ridge ending may well be confused with an unusually large gap between two consecutive sweat pores. In addition these techniques do not work for porous material that has been exposed to water since the amino acids are water soluble and will be removed from the fingerprint residue.
Fingerprints developed by physical developer on porous surfaces that have been exposed to water are also well documented in the literature. More specifically, the fingerprints develop a dark grey silver residue on their ridges which is visible to the naked eye providing the background is not heavily patterned or dark.
The main drawback with physical developer is that the technique is not fluorescent and therefore the ability to remove any complicated background patterns by using filters that only allow the wavelength corresponding to the fluorescence through can not be applied. In addition physical developer is a very expensive technique approximately $275 (canadian) for 4 litres and it has a shelf-life of about one year.
The application of europium for visualising fingerprints on paper has been described in the literature by E. R. Menzel (Fluorescent Metal-Ruhemann's Purple Coordination Compounds: Applications to Latent Fingerprint Detection, Journal of Forensic Sciences, JFSCA, Vol. 35, No. 1, January, 1990, pp. 25-34). Latents were reacted with ninhydrin which yields the product Ruhemann's Purple. This compound was then reacted with europium ions as opposed to zinc ions to form a weakly fluorescent chelate when viewed under laser light. This method is very inefficient and expensive, as time-resolved imaging is required to minimize the background fluorescence.